Halide glass compositions

ABSTRACT

This invention relates to halide glasses which have particular utility as hosts for rare earth elements in order to provide optical amplification by laser activity. The glasses are characterized in that the metal content is similar to conventional ZBLAN glasses except that it has been discovered that the replacement of Al by Y and In and the use of more than one alkali metal fluoride, e.g., NaF, CsF and LiF, has synergistic benefits. The synergistic benefits are good lasing performance (due to the low content of aluminum) and good stability in spite of the low content of aluminum. Pr 3+   constitutes a good lasing species for amplifying telecommunications signals at 1300 nm using pump radiation at 1020 nm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to halide glass compositions and moreparticularly to halozirconate glass compositions which, have goodproperties as hosts for rare earth elements as lasing dopants.

2. Related Art

It has long be recognised that the rare earth elements displayfluorescence and this fluorescence can be utilised in the form of lasingeither for the generation of optical signals or for the amplification ofoptical signals. Usually the lasing species is a trivalent ion of a rareearth element. In particular the trivalent ion Pr³⁺ (praseodymium)constitutes a lasing species for providing radiation at 1300nm. Thisproperty is of interest because optical telecommunications uses signalsat 1300nm and the ion Pr³⁺ is capable of amplifying such signals bylaser action. It will be apparent that, in order to take advantage ofthis property, it is necessary to provide the active species in asuitable waveguiding structure, eg. a fibre waveguide.

While the element Pr is of particular interest for telecommunications itshould be recognised that all the rare earth elements are capable oflasing at a variety of different wavelengths for a variety of differentpurposes. In other words the lasing properties extend throughout thegroup of rare elements and there is, therefore, general interest inproviding all of the rare earth elements as lasing species in suitablehost glasses.

The halide, eg fluoride, glasses have been recognised since 1978 and awide range of compositions have been reported and their propertiesstudied. It has been recognised that the halide glasses form good hostsfor the rare earth elements as lasing species but the identification andselection of compositions having favourable properties remainsdifficult. In particular the prior art has failed to identify the glasscompositions capable of lasing at 1300 nm with sufficient efficiency foruse in telecommunications networks. This invention relates tocompositions which have good properties. It is now convenient to discussthe properties of the class required in a lasing device such as a fibreamplifier. These properties will be considered under three differentheadings.

General Glass Properties

It is important that all glasses shall remain in the glass state, ie.they shall not devirtify under condition of use. It is also importantthat the glasses shall not be subject to crystallisation which might beconsidered as incipient devitrification. In addition it is alsonecessary that the compositions shall be suitable for use in glassforming and further processing. In particular it is necessary that acomposition be stable in the melt, that it shall be capable ofwithstanding practical cooling rates and the conditions necessary forfibre forming, eg. during the pulling of a fibre preform into a fibre.It will also be apparent that chemical stability of the various glasscomponents is important, eg it is desirable to avoid water solubleingredients and, even more important, to avoid hygroscopic ingredients.

Attenuation

Lasing devices usually include waveguiding structures and it is clearlyimportant to avoid unnecessary attenuation of either the signalwavelength or the pump wavelength. The requirement for low attenuationmeans that it is desirable to avoid components which have unnecessarilyhigh absorptions at wavelengths of interest. It is also necessary toavoid scatter which emphasises some of the fundamental glass properties,ie. that the glass shall not form crystals even on a small scale.

Host Properties

It also appears that there is interaction between the host glass and thelasing species. For example, the lasing species may undergo what isoften called "non-radiative decay". This implies that the lasing specieslooses energy other than by the intended lasing transitions.Non-radiative decay represents a loss of energy and it is, therefore, anundesirable effect. It appears that the host glass may participate innon-radiative decay either in the sense that it may assist thisundesired effect or help to inhibit it. Nevertheless, whatever thereason, it is established that the host glass can affect the efficiencyof the lasing process and it is desirable to select the host so as toachieve good lasing efficiencies.

The hosting properties of the glass appear to have substantial effectsupon the efficiency of a laser, eg. the ratio of signal power output topump power input. This efficiency is of substantial importance intelecommunications because it may define the available gain of anamplifier. In experimental work, it is often convenient to utilise thelifetime of the excited state as a measure of the efficiency; the twoquantities can be regarded as proportional to one another. In sometheoretical papers it is considered that the multi-phonon absorption ofthe host affects the lifetime of the excited state and hence theefficiency of lasers based thereon.

It is important to recognise that the selection of a lasing composition,and especially the host glass, must take into account all of thesefeatures. Thus it is not necessarily appropriate to select ingredientssolely on the basis of their effect upon the lasing performance if thesecomponents are liable to give rise to glass instability and highattenuations (which high attenuations may be the result of glassinstability). In other words, selecting on the basis of one desirablefeature is unlikely to produce acceptable results if this selection isaccompanied by adverse effects.

It has been mentioned that the prior art has disclosed and evaluated avery wide range of different halide (fluoride) glasses. This rangeincludes a well recognised group usually known as fluorozirconates. Thissub-group of fluoride glasses has been recognised because its membersperform well in respect of all of the above features. The chemicalcomposition of the fluorozirconate glasses will now be described.

The major component is ZrF4 which usually constitutes about 40-65 mole %of the total composition. In some variants the content of ZrF4 isreduced in order to adjust the refractive index, eg. by incorporatingPbF2 or HfF2. (Refractive index adjustment is important in the design ofwaveguiding structures). A fluorozirconate composition usually containsabout 10-39, eg. 15-25, mole % of an alkali metal fluoride, usually NaF.In addition, the composition often contains a substantial amount, eg.10-mole % of BaF2 with smaller amounts, eg. 2-6 mole %, of LaF3 andAlF3. It is emphasised that the halide content of a fluorozirconateglass is entirely fluoride. In the case of a lasing composition, thefluorozirconate host will also contain up to 4 wt % of the cation of arare earth metal, eg 0.001 to 0.1 wt % (ie 10-1000 ppm. wt) of Pr³⁺.

SUMMARY OF THE INVENTION

It has now been unexpectedly discovered that the aluminium, which isconventional in halozirconate glasses, tends to affect adversely thefluorescence and lasing properties of dopants such as rare earth metals,eg Pr³⁺. This invention, which is more fully detailed in the claims,consists in the use of halozirconate glasses which contain less than0.2% mole of aluminium, preferable less than 0.1% mole of aluminium andmost preferably no aluminium.

The use of low, as defined above, aluminium concentrations has anadverse effect on the stability of the glass and this inventioncomprises two modifications each of which improves the stability of lowaluminium glasses. Either one of the said modifications by itself isbeneficial but it is preferred to incorporate both, especially inglasses which contain no aluminium.

According to the first modification the composition contains at leasttwo, and preferably three, halides of different alkali metals, eghalides of Na, Li and Cs.

According to the second modification the composition contains a halideof indium or yttrium, preferably both.

Compositions which contain only a small amount, e.g. 0.05 to 0.15% ofaluminium halide will show a small loss of lasing performance but the Alhelps to stabilise the composition and only one of the two modificationsmay be needed, e.g. the use of In and/or Y (with only one alkali metal).

To obtain maximum lasing performance it is preferable to have noaluminium present and, in order to achieve good stabilities, it isrecommended to use both modifications, e.g. to have both In and Yhalides present and also to have halides of at least two of the alkalimetals Na, Cs and Li present.

Where In and Y are both present the relative molar quantities arepreferably:

Mole ratio In: Y=1:3 to 3:1, eg 1:1

Where Na and Cs are both present the relative molar quantities arepreferably:

Mole ratio Na:Cs=5:1 to 1:3, especially 3:1 to 1:2, eg. 1:1.

The halide content of the low aluminium glass is conveniently allfluoride. However particularly good lasing has been achieved wherein upto 10% weight, eg 1-5% weight preferably 3-4% weight of the total lostcomposition is produced as chloride with the remainder of the halideprovided as fluoride.

The invention includes not only the novel glasses described above butalso:

(i) waveguiding structures, eg fibres, made from the glasses, especiallywaveguiding structures having path regions made of the glasses; and

(ii) signal generators and photonic amplifiers utilising the glasses tosupport lasing activity.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Three compositions in accordance with the invention will now bedescribed by way of example. In addition, three more compositions willalso be described to provide a basis of comparison. These sixcompositions are defined in Table 1.

These compositions were made by conventional preparative techniques,e.g. mixing the powered ingredients in a crucible, melting and casting.All processes were carried out under clean, dry atmospheres such as N₂or A. Oxygen may be present during part of the melting. A suitabletechnique is described in EP 170380. A small amount of NH4H-2 was addedto fluorinate residual oxides (˜0.5 g). The batch was then heated in aPt/Au crucible under flowing N2 at 400 C. for 1 hr, the temperature wasraised to 850 C. and the glass was further heated under O₂ for 2 hrs tooxidise the melt, this is followed by a further 1 hr at a lowertemperature of 670C., before casting. The casting was performed underpartial vacuum lower with a flow of dry N2 to prevent bubble formation.A partial vacuum also applied during the glass melting process whilstthe melt was at 670° C.

Similar preparative technique apply to mixed halide (fluaoro/chloro)glasses, e.g a suitable proportion of the reactants is provided aschloride. Fluorinating agents, e.g. NH4HF2, should be avoided as thereis a risk of converting chloride into fluoride.

                                      TABLE 1    __________________________________________________________________________    Fluorozirconate Compositions (mole %)    Acronym         ZrF4             BaF2                LaF3                    AlF3                        YF3 InF3                               NaF CsF λ    __________________________________________________________________________    ZBLAN         52  20 4   4   --  -- 20  --  7.05    ZBLYIN         52  20 4   --  2   2  20  --  7.23    ZBLAC         55  22 4   4   --  -- --  15  7.10    ZBLYIC         55  22 4   --  2   2  --  15  7.36    ZBLANC         52  20 4   4   --  -- 10  10  7.10    ZBLYINC         52  20 4   --  2   2  10  10  7.35    __________________________________________________________________________

The compositions ZBLAN, ZBLAC and ZBLANC all contain 4% of ALF₃ and,therefore, they represent the prior art. ZBLYIN and ZBLYIC represent thefirst modification whereas ZBLY:NC represents a preferred embodimentshowing both modifications.

In addition to the ingredients specified in Table 1 the compositionsalso contain 500 ppmw of Pr3+, based on the other ingredients. The Pr3+is an active dopant capable of supporting lasing and amplifyingactivity.

The columns headed λ in Tables 1 gives the wavelength in micrometerswhich represents the limit of infra red transmission for the relevantcomposition. The composition will transmit at wavelengths shorter than λbut attenuation is very high at wavelengths longer than λ. (λ is usuallyknown as the "infra red cut off").

Some theorists consider that the multi-phonon absorption properties of aglass affect many of its optical properties. For example it isconsidered that multi-phonon absorption affects the infra red cut off(ie. λ as quoted in Table 1) and also the interaction with lasingprocesses taking place within the glass. We note that in Table 1 theA1-free compositions exhibit a longer infrared cut off then thetraditional fluorozircontate glasses.

Before quoting numerical values it is appropriate to establish certainqualitative comparisons.

The halides of Zr, Ba and La are the primary glass forming constituentsand these three halides constitute about 75 mole % of the composition.These three metals can be partially replaced by other metals, eg. Hf, inorder to adjust refractive index to provide waveguiding structures.

Conventional fluorozirconate glasses contain AlF3 but this can bereplaced with halides of indium and yttrium with beneficial effects asset out quantitatively below.

Alkali metal halides are required to provide a stable glass compositionand this is usually provided as NaF. The replacement of Na by Cs hasbeen proposed but (in conventional systems) we have found that thisreplacement can result in a significant decrease in the stability of theglass. In conventional systems, therefore, it is not considereddesirable to replace Na by Cs.

Three important performance parameters were measured for the glasscompositions quoted defined in Tables 1 and the results are quoted inTable 2.

                  TABLE 2    ______________________________________    Acronym   Life (μs)  Stability                                    Tx-Tg    ______________________________________    ZBLAN     107           6.74    92    ZBLYIN    126           3.41    83    ZBLAC     106           4.21    70    ZBLYIC    120           2.48    70    ZBLANC    108           3.26    71    ZBLYINC   134           6.77    90    ______________________________________

The column headed "life" in Table 2 gives the fluorescence lifetime ofthe Pr³⁺ in the specified host. The fluorescence was stimulated by pumpradiation at 1020 nm provided with an Ar+ pumped Ti: sapphire laser. Thelifetime specifies the rate of decay of fluorescence after the pump hasbeen switched off. The fluorescence is at 1300 nm and it corresponds tothe lasing transitions (¹ G₄ -³ H₅) which would be needed in atelecommunications amplifier operating at this wavelength. Theefficiency of the laser is proportional to the lifetime.

The other two columns, headed "stability" and "T_(x) -T_(g) "of bothrelate to the stability of the glass. More specifically three glassparameters are involved, these are:

T_(g) =the glass transition temperature,

T_(x) =temperature of onset of crystallisation.

T_(p) =temperature of peak crystallisation

The "stability", represented as S, is calculated as:

S= (T_(p) -T_(x)) (T_(x) -T_(g)) !/T_(g)

T_(g) and T_(x) and T_(p) were all read off from differential scanningcalorimetry curves obtained using an isochronal heating rate of 20°C./minute. The stability and (T_(x) -T_(g)) are parameters whichrepresent the thermal stability of the glass and the higher theparameter the better. However the overall properties which make a glasssuitable for using in a waveguide are even more complicated and thestability parameter represents only one important feature of the overallperformance.

Table 2 shows that simply replacing Al by In an Y in the traditionalfluorozirconate glass (ZBLAN) has the desirable effect of increasing thePr³⁺ 1 G₄ lifetime.

However, this is accompanied by a reduction in glass stability. Thusfibres containing a core of ZBLYIN glass will have greater optical loss,due to the formation of higher concentration of scattering centres, thana traditional ZBLAN fibre. Table 2 also shows that replacing Na with Csin both the traditional fluorozirconate glass and the Al-freecompositions has little effect on the lifetime of the dopant but thisreduces the stability of the glass further. Most surprisingly, however,we have found that by mixing Na and Cs the Al-free fluorozirconate glassstabilised. Furthermore, there seems to be a synergistic enhancement ofthe donant lifetime. Thus a fibre containing a doped core made ofAl-free fluorozirconate glass containing mixed alkali metal fluoridesand also fluorides of In and Y will have similar levels of optical lossas a fibre composed of traditional fluorozirconate glass. Thus thebenefits arising from an enhanced lifetime of the dopant can beexploited without degradation of the device performance through enhancedoptical loss.

The glass compositions according to the invention contain Pr⁺³ as thelasing dopant and they are useful in amplifiers for telecommunicationssignals at a nominal wavelength of 1300 nm. Such signals have a bandwidth which usually extends as low as 1260 nm and/or as high as 1340 nm.Signals with this nominal wavelength can be amplified using the lasingtransition ¹ G₄ →³ H₅ and the Pr⁺³ is pumped using a nominal wavelengthof 1020 nm, e.g. using the band 960-1080 nm.

In the compositions specified in Table 1 (above) all of the halidecontent is provided as fluoride. The results quoted in Table 2 anddiscussed above therefore relate to all-fluoride systems.

It has been found that changing some of the fluoride into chloride hasbeneficial effects. The amount changed should not result in the chloridecontent of the total composition missing above about 10% by weight.Where chloride is present, concentrations of 1-5% by weight, eg 3-4%weight, based on the total composition are preferred.

In the ranges specified, the presences of chloride appears to enhancethe lasing benefits, eg the fluorescence lifetime of Pr³⁺ is increased.

However particularly good effects have been achieved for Pr³⁺ in a hosthaving the following features:

(a) No aluminium content

(b) yttrium and indium, preferably in ecuimolar quantities, to replacethe aluminium,

(c) alkali metal content provided as sodium and caesium preferably inequimolar quantities

(d) chloride content of 2-5% weight.

These four features interact so as to give not only high fluorescencelifetimes but good stability.

In order to illustrate this effect the (all fluoride) compositiondesignated ZBLYINC in Tables 1 and 2 was modified by providing some ofthe fluoride content as chloride such that the amount of chloride is upto 5% weight based on the total composition.

The effects on the lifetime and the stability (as defined above) are asfollows:

    ______________________________________    % CI            L      S    ______________________________________    0               134    6.77    2               140    5.41    3               147    10.56    4               153    13.03    ______________________________________

The line 0% corresponds ZBLYINC as given in Table 2 above (ie the bottomline thereof). The other lines show the effect on changing the statedamounts of fluoride into chloride. It will be noticed that the lifetimeincreased with chloride concentration but the stability for 2% chlorideis less than for all fluoride. However 3% and 4% of chloride gave goodstabilities as well as a high fluorescence lifetime.

In addition the composition ZBLYIN of Tables 1 and 2 (ie line 2) wasmodified to contain 4% weight of chloride based on the totalcomposition. The fluorescence lifetime increased from 126 to 163 whichis an excellent value. The stability was only slightly increased, iefrom 3.41 (all fluoride) to 3.55 (mixed halide).

We claim:
 1. A halide glass corrosion which composition consists of ahost glass 0.001 to 4 weight % based on the host glass composition of anactive dopant, wherein the host contains at least one halide of In and Yan at least 1 alkali metal halide characteristic in that:(i) the totalamount of the halide of In and Y is 1-10% and aluminium halide at aconcentration between 0.0% and 0.2%; and (ii) tile total amount ofalkali metal halides is 10-39% wherein all the percentages are molarpercentages based on the total host glass composition.
 2. A glasscomposition as in claim 1, wherein the total amount of alkali metalhalides consists of at least 1% of each of a plurality of differentalkali metal halides, said halides being derived from alkali metals,said alkali metals being selected from Na, Cs and Li.
 3. A glasscomposition as in claim 1 wherein the amount of aluminium issubstantially zero and the composition contains at least 1% of indiumhalide and at least 1% of yttrium halide.
 4. A glass composition as inclaim 1 which contains, in addition to the halides of alkali metals andAl, In, and Y, as specified, at least 40% of a zirconium halide at least10% of a barium halide and at least 2% of a lanthanum halide.
 5. A glasscomposition as in claim 1 wherein said active dopant is a trivalent ionof a rare earth.
 6. A glass composition as in claim 5, wherein theactive dopant is praseodymium and its concentration is 0.001 to 0.1 wt%.
 7. A glass composition as in claim 1 which additionally contains atleast one halide of lead, hafnium and thorium.
 8. A glass composition asin claim 1 wherein the halide content is made up entirely of fluorideand chloride, the chloride content being less than 10% wt based on thetotal composition.
 9. A glass composition as in claim 8, wherein thehalide content is all fluoride.
 10. A composition as in claim 9, whereinthe host glass consists of the percentages α, β, γ, δ, Φ, ψ, θ, ω, ζ, μand π of the ingredients identified in the following tabulation:

    ______________________________________    α   %                 Zr F.sub.4    β    %                 Ba F.sub.2    γ   %                 Ca F.sub.3    δ   %                 Pb F.sub.2    Φ     %                 Hf F.sub.4    Ψ     %                 Li F    θ   %                 CsF    ω   %                 NaF    ζ    %                 Al F.sub.3    μ      %                 Y F.sub.3    π      %                 InF.sub.3    ______________________________________

wherein each of the ingredient percentages satisfies the quantitativerelationships defined in tabular form below:

    ______________________________________    minimum           percentage         maximum     MIN!             amounts             MAX!    ______________________________________    60%      ≦ α + β γ + δ + φ                                  ≦                                         90%    45%      ≦   + φ   ≦                                         90%    45%      ≦ α + δ                                  ≦                                         90%    10%      ≦ ω + θ ω                                  ≦                                         39%    2:1      ≡  (Ψ + θ):ω                                  ≡                                         1:2    1%       ≦ ζ + μ + π)                                  ≦                                         10%    0%       ≦ ζ      ≦                                         1%    0:10     ≡  ζ:(μ + π)                                  ≡                                         1:10    ______________________________________


11. A composition as in claim 10, wherein at least two of said hostglass ingredient percentages ψ, θ and ω are greater than
 5. 12. Acomposition as in claim 10, wherein each of said host glass ingredientpercentages ψ, θ and ω is greater than
 3. 13. A composition as in claim10 wherein each of said host glass ingredient percentages μ and π isgreater than
 1. 14. A composition as in claim 10, wherein said hostglass ingredient percentage α is greater than 45 and less than
 55. 15. Acomposition as in claim 10 wherein said host glass ingredient percentageζ0.
 16. A fibre waveguide having a fibre core made of a glasscomposition as specified in claim
 1. 17. An optical amplifierincluding:a waveguide as in claim 16, an input port for connecting saidwaveguide to receive optical signals; and a pump for providing pumpradiation into a core of the waveguide so as to provide power to sustainoptical amplification by lasing activity.
 18. A fluorochlorozircontateglass composition for use in an optical signal amplifier or in a laserwhich composition consists of a host glass and 0.001 to 4% wt, based onthe host glass compositions of praseodymium and the host glass containshalides of the elements Zr, Ba, La, Y, In, Cs and Na subject to theconditions that:(i) the composition contains no aluminium; and (ii) thehalide content is provided as 1-5% wt of chloride and that the remainderof said halides is provided as fluoride.
 19. A composition as in claim18, wherein the halide content is 3-4% wt.
 20. A composition as in claim16 having a mole ratio of In: Y within the range 2:3 to 3:2.
 21. Acomposition as in claim 18 having a mole ratio Na:Cs within the range5:1 to 1:3.
 22. A composition as in claim 18, which additionallycontains at least one halide selected from halides of Hf and Li.
 23. Acomposition as in claim 18, which only contains halides of the elementsZr Ba, La, Y In, G and Na.
 24. A method of amplifying telecommunicationsignals at a nominal wavelength of 1300 nm, said methodcomprising:providing said telecommunication signals into a glasscomposition as in claim 1 wherein an active dopant is praseodymium andsimultaneously providing into said glass composition pump radiation at anominal wavelength of 1020 nm whereby said pump radiation excites saidpraseodymium into an inverted state to generate more photons at 1300nmwhereby said telecommunication signals are amplified.